Electronic marker devices, of the type having an inductance-capacitance tuned circuit, are known. Such devices comprise an air-core inductance provided by a wire coil connected in parallel with a capacitor. Optimum operation for relocation of such a marker requires vertical orientation of the longitudinal axis of the coil. A core-wound coil, also herein referred to as a solenoid coil, may be replaced in the tuned circuit by a substantially planar coil extending horizontally outward from a central axis. In this case, positioning of the marker beside a utility line preferably sets the coil in a horizontal plane with its central axis in vertical orientation. U.S. Pat. No. 4,712,094 provides a solution to earlier problems in achieving desired coil positioning using a marking device containing a floating tuned circuit that adopts the correct orientation regardless of initial placement of the marker beside a buried line.
Optimum orientation is important for coupling the coil of a passive marker with the coil of a locator device during relocating of a buried marker. Any difficulty associated with relocating a marker will complicate the process of reentry for maintenance or repair of a particular portion of the line. A locator of passive electronic markers normally emits a pulsed electromagnetic field to energize the marker. Between pulses, the marker releases its stored energy, at its resonant frequency, producing its own electromagnetic field. The resonant signal from the marker may be detected by switching the locator from transmitting mode to receiving mode. In the receiving mode, the maximum detectable signal usually identifies the position of a selected marker which lies adjacent to the portion of the utility line or line component requiring inspection. As previously discussed, tuned circuits floating inside a container adopt an orientation for optimum coupling of electromagnetic fields produced by the locator circuit and the marker tuned circuit. U.S. Pat. No. 5,699,048 describes effective coupling of signals using an orthogonal arrangement of plural tuned circuits. This tuned circuit arrangement produces a multidirectional response pattern that is more rapidly detected than that of poorly orientated single circuits.
Rapid, successful detection of the location of a marker reduces the amount of time for line inspection. The distance between a marker locator and a passive electronic marker will affect how quickly the position of the marker may be correctly identified. Successful marker detection depends upon the sensitivity of the marker locator and the range of detectable signal associated with the marker's resonating tuned circuit. Improvement in the range of a detectable signal will add further value to passive, essentially self-orienting marker devices.